For years, the hard disk drive industry has focused on longitudinal magnetic recording to record data on disks. In longitudinal recording, the direction of the magnetic charge for each data bit is aligned horizontally to the disk that spins inside the hard drive. More recently, perpendicular magnetic recording (PMR) has become the favored technology. In PMR, the direction of magnetic charge for each data bit is aligned vertically to the spinning disk, providing the ability to substantially increase aerial density by providing more data on a disk than is possible with conventional longitudinal recording.
New technologies are being developed to further increase aerial density by reducing the size of the magnetic grains that comprise the data bits in the recording layer on the surface of the disk. This reduction in the size of magnetic grains requires a corresponding increase in the magnetic anisotropy in the recording layer to maintain thermal stability. Any increase in magnetic anisotropy, however, requires a stronger write field to switch the magnetic grains and write to the disk. Today, the ability of write heads to produce sufficient write field strength is one of the limiting factors in reducing the magnetic grain size to increase aerial density.
One possible solution is PMR with high frequency assisted writing using a spin torque oscillator (STO). This type of recording, also called microwave assisted magnetic recording (MAMR), applies a microwave field from the STO to the magnetic grains embedded in the recording layer on the surface of the disk. The microwave field may have a frequency close to the resonance frequency of the magnetic grains to facilitate the switching of the magnetization of the grains at lower write fields than would otherwise be possible without assisted recording.
The STO may be located between the write pole tip and the trailing shield of the PMR write head. The STO is a multilayer film stack that includes a polarization layer and a field generating layer. When an electrical current is applied to the STO, the polarization layer generates a spin-polarized current. The spin-polarized current is used to excite magnetic oscillations in the field generating layer and thereby generate a microwave field useful for MAMR applications.
The conventional STO has a rectangular prism shape. However, this shape does not provide sufficient control over the STO frequency (i.e., magnetic oscillations) with applied current to optimize the microwave frequency and the write field to the media requirements. A cylindrical shaped STO, on the other hand, provides better frequency sensitivity with applied current, but has a very narrow stripe height lapping range that will yield an acceptable microwave field strength and magnetic write width.